One example of a heat accumulator of the general type of concern is shown in DE 40 41 626. To avoid mixing of warm fluid with cold fluid in the accumulator, this heat accumulator uses an intermediate accumulator into which cool fluid is introduced during discharge of the accumulator. The introduction of fluid into the intermediate accumulator forces warm fluid from the accumulator. The intermediate accumulator, for example, can be of a flexible bellows or piston-cylinder design. Such an accumulator is a representative of the type often referred to as a membrane accumulator, and has recently been considered to an increasing extent for use in vehicles because it has the capability of supplying large amounts of heat by avoiding mixing of cold fluid and warm fluid.
Another type of heat accumulator, which is also intended for use in vehicles, is a latent heat accumulator, which includes an inner vessel, an accumulator core in which an accumulator medium, e.g., an accumulator salt, is situated, and an outer vessel with insulation provided between the core and the outer vessel.
Advantageously, a latent heat accumulator has a large storage capacity. Offsetting this advantage, is a more limited discharge rate. Further, latent heat accumulators are quite costly to manufacture. They pose problems in terms of disposal because of the polluting nature of the storage medium. Membrane accumulators in this respect are preferred, but are still too expensive because of the membrane and the additional required plumbing. Moreover, membrane accumulators do not have an acceptable useful life. Another significant drawback of both accumulator types is that their geometry is not easily modified to accommodate irregularly-shaped installation spaces within the engine compartment of a vehicle. This is becoming increasingly important because the available space in vehicle engine compartments is diminishing as the number of components incorporated within engine compartments increases.
Another type of heat accumulator is a layered accumulator which typically is used in households and which exploits the higher density of a fluid at cooler temperatures by introducing cool fluid into a lower storage space and withdrawing warm fluid from an upper storage space, with the least possible mixing of the cool and warm fluids. Layered accumulators typically have a storage volume of many cubic meters, which is much larger than the previously discussed heat accumulators that are intended for use in vehicles. Further, layered accumulators are rarely, if ever, fully discharged. Accordingly, layered accumulators are typically subject to different constraints than heat accumulators intended for use in vehicles.